(1) Field of the Invention
The present invention relates to coatings for metal surfaces that protect the surfaces from corrosion, and more particularly to radiation curable corrosion-responsive coatings for metals and components of such coatings.
(2) Description of the Related Art
In the United States approximately $300 billion per year in direct costs is lost due to metallic corrosion. More than one third of costs are considered avoidable using existing know-how and technology. Coatings are the primary and most economical means for controlling the corrosion of metals. The key factors that influence corrosion are the type of metal being used (aluminum, steel, copper, etc.) and the environment to which the metal is exposed (pH, temperature, humidity, chemicals, etc.).
Current strategies for corrosion protection include: dispersion of pigments in coating systems which act as passivating agents, including strontium chromate, zinc chromate, zinc phosphate, barium metaborate, etc.; dispersion of pigments in coating systems which provide cathodic protection (e.g., zinc dust which acts as a sacrificial anode); and the provision of mechanical protection by applying thick multilayer coating systems such as epoxies, urethanes, acrylics and rubbers, which are impervious to moisture and chemical ingress. What is lacking with current coating strategies, however, is an environmentally friendly coating system that prevents corrosion and pitting even in the presence of pinholes or scratches.
Problems with a passivation coating, such as chromium VI (the form of chromium commonly used in aerospace coatings), include the fact that chromium is a carcinogen and federal, state and local agencies have issued regulations that limit or prohibit the use of chromated materials. OSHA regulates the amount of hexavalent chromium to which workers can be exposed, and has proposed reducing the Permissible Exposure Limit (PEL) from the current 50 micrograms/m3 to less than 1 microgram/m3. OSHA's proposed PEL would severely impact the use of hexavalent chromium throughout the aerospace sector.
The primary function of barrier coatings is to prevent the ingress of water and salts. However, such coatings often lack pinhole protection. Any pitting or scarring that penetrates the underlying structures can lead to catastrophic corrosion damage. To compensate for the lack of pinhole protection, multiple layers are applied.
Sacrificial coatings are designed to corrode and cathodically protect the underlying structure. These coatings wear more readily, and the layer thickness and its associated weight can negatively impact structural design.
Epoxy primers containing chromate with polyurethane top coats are widely used for corrosion protection in the aircraft industry. Strontium chromate coatings, while extremely effective, are under significant pressure to be eliminated because of their carcinogenic classification. In addition, chromic acid anodizing and other chromium conversion coating systems are also commonly employed to enhance corrosion protection and also adhesion of the epoxy primer coating to aluminum.
The need for anti-corrosion coatings, which are pinhole and scratch tolerant, coupled with growing environmental concerns involving heavy metals, such as hexavalent chromium, has led to new coating strategies. In one area, coatings that employ intrinsically conductive polymers (ICPs) have been reported. The first documented observations of corrosion protection of steel by polyaniline were reported in 1981 by Mengoli, et al., Appl Polymer Sci., 26:4247 (1981). Since then, numerous papers have been published on the corrosion protection of carbon steel (Kinlen, et al., Corrision, 58:490 (2002)), stainless steel (Casparac et al, J. Electrodhem. Soc., 148:B138 (2001)), iron (Beck, Metalloberflacche, 46:177 (1992); and Beck, et al., Electrochimica Acta, 39:229 (1994)), titanium, copper (Brusic, et al., J. Electrochem. Soc., 144:436 (1997), and aluminum alloys (Gelling, et al., Prog. Organic Coatings, 43:149 (2001)), with ICP's. Two comprehensive review articles have been published. See, e.g., McAndrew, Trends in Polymer Science, 5:7 (1997); and Spinks, et al., J. Solid State Electrochemistry, 6:85 (2002).
Other work has led to the use of “smart” coatings, which contain materials designed to release a corrosion-inhibiting species on demand during corrosion. For example, in WO 90/10095, Wallace reports a polymer coating, where the polymer is preferably an electrically conductive oligomer, such as polypyrrole, that contains ions such as chromate, EDTA, and others, which are released in response to contact with ionic species that are the product of the oxidative/reductive chemical reactions that occur during corrosion. In U.S. Patent Publication 2002/0197468A1, Sinko identifies corrosion-inhibiting organic pigments, such as 2,5-dimercapto-1,3,4-thiadiazole (DMTD), and others, that demonstrate “throw power” (an ability to maintain a scribed line on a coated metal surface free of corrosion in a corrosive environment). In U.S. Pat. No. 6,139,610, Sinko describes certain inorganic and organic pigment compositions as being effective corrosion inhibitors, again with DMTD being mentioned. In another publication, Sinko identified certain inorganic materials as being potential replacements for chromates. Sinko, J., Prog. in Org. Coatings, 42:267-282 (2001).
Although epoxy-based coatings predominate in commercial corrosion prevention applications, other polymeric systems are suggested. One drawback of many polymeric systems, however, is the use of solvents, or the formation of water or gas during curing. The removal of the solvents, water, or gas from the coating as it cures leaves holes, pits, and voids in the cured film, through which water, oxygen and other corrosive elements can penetrate to reach the metal surface.
Radiation-curable polymer systems, such as UV-curable resins, can be formulated to be solvent-free, and have been used to form films that contain various chemicals. Kim, Y-B, et al., Polymers for Advanced Technologies, 13(7):522-526 (2002), have reported UV cured transparent films containing conductive microgels coated with polyaminiline/dodecylbenzenesulphonic acid (DBSA). Others have reported the corrosion-protective effects for aluminum of polymeric blend coatings containing either polyaniline, polypyrrole, or other polymers, and UV-curable urethane acrylate binders. Vang, C. et al., Polymer Preprints, 43 (1), Spring 2002, Papers presented at the ACS meeting held Orlando, Fla., Apr. 7-11, 2002, ACS Div. of Polymer Chemistry. In Japanese unexamined patent JP 11/172,103, aniline-type resin compositions are cured with UV radiation. The polyaniline in the cured films is doped with a sulphone compound, and the film is reportedly useful as an antistatic agent.
Despite the availability of radiation-cured polymeric systems, they have not been widely used to form corrosion-resisting coatings, and certain problems remain to be resolved. It is known, for example, that the corrosion-inhibiting compound DMTD is itself a strong UV absorber. It is unclear, therefore, whether such a material could be included as a component in a UV-cured resin system at a level that would be useful for corrosion inhibition without interfering with the curing of the coating.
Accordingly, therefore, it would be useful to provide corrosion-inhibiting methods and compositions that provided effective corrosion protection for metal surfaces. It would also be useful if such methods and compositions supplied corrosion-inhibiting agents in response to actual corrosion on a metal surface, and if they provided corrosion protection for pinholes and scratches that might occur on the metal surfaces. It would also be useful if such methods and compositions retained their effectiveness through normal weather exposure.